Process of preparing hydrogen fluoride from fluosilicic acid



July 20, 1965 G. M. BURKERT ETAL 3,195,979

PROCESS OF PREPARING HYDROGEN FLUORIDE FROM FLUOSILICIC ACID Filed Dec. 18, 1961 United States Patent O 3,195,979 PROCESS F PREPARHNG HYDRGEN FLUG- RIBE FRM ELUGSMCHC AClD George M. Burkert and Arthur N. Baumann, Lakeland, Fla., assignors to lnternationai Minerals @a Chemical Corporation, a corporation of New York Filed Der. 18, 1961, Ser. No. 150,292 Claims. (Cl. 23-153) This is a continuation-in-part application of application Serial No. 861,782, tiled December 24, 1959, now abancloned.

The present invention generally relates to a process for the production of fluorine compounds from uosilicic acid. More particularly it relates to a process for preparing hydrogen fluoride from fluosilicic acid.

Fluosilicic acid is a commercially available material of relatively low cost. Large amounts of uosilicic acid are produced as a by-product of the fertilizer industry.

Some fluosilicic acid is used in the production of aluminum fluoride and synthetic cryolite and because of its y ready availability and relatively low cost, new uses for the acid are being investigated. The process of the present invention provides a novel process for producing hydrogen iluoride from iiuosilicic acid. Large quantities of hydrogen uoride or hydroliuoric acid (HF) are used in the production of aluminum iluoride, cryolite and other metal uorides. The process of this invention may, when desired, also be used to prepare potassium bifluoride and sodium biiluoride from fluosilicic acid.

It is an object of the present invention to provide a new and novel process for the production of fluoride compounds from fluosilicic acid.

It is another object of the invention to provide a process for preparing hydrogen uoride from fluosilicic acid.

It is a further object of the present invention to provide an integrated and autogenous process for preparing hydrogen fluoride from liuosilicic acid which includes the step of reacting a uoride of ammonia with an alkali metal iiuoride to prepare an alkali metal biuoride.

These and other objects and advantages of the present invention will be apparent from the detailed description of the invention.

Generally described, the present invention is a process for preparing hydrogen liuoride from fluosilicic acid which comprises the following steps:

(a) Reacting fluosilicic acid with ammonia to form ammonium iiuoride and silica,

(b) Separating said silica from said ammonium iluoride,

(c) introducing said separated ammonium fluoride and an alkali metal fluoride into a treating zone wherein said ammonium fluoride and said alkali metal uoride are subjected to treating conditions to react a fluoride of ammonia with said alkali metal fluoride to form an alkali metal biuoride,

(d) Separating said alkali metal biiluoride from the mixture resulting from step (c),

(e) Heating said separated alkali metal biiluoride to a temperature at least suiiiciently high to produce an alkali metal biuoride substantially free of ammonia and ammonium iluoride and below that at which substantial decomposition of said alkali metal biuoride is eiiected,

(f) Heating the alkali metal biiluorideV substantially free of ammonia and ammonium iiuoride from step (e) to a temperature sufficiently high to decompose said alkali metal bifluoride to form hydrogen uoride and alkali metal iiuoride,

(g) Separating said alkali metal iluoride and said hydrogen fluoride produced by the heating in step (f) (h) Recycling said alkali metal uoride separated in step (g) to step (c), and

"ice

(i) Recovering said hydrogen liuoride separated in Step (s).

The instant process offers an economical and novel method of preparing hydrogen iiuoride. An aqueous solution of iluosilicic acid is the primary raw material of the process and the primary products are silica and hydrogen fluoride, both of which have known commercial utility. The ammonia and alkali metal fluoride used in the process are recycled within the process and only make-up quantities, to take care of process losses, are necessary. The overall process, therefore, represents an integrated and interdependent series of process steps for producing hydrogen iluoride from fiuosilicic acid. Whereas prior art processes for the production of hydrogen uoride usually require a plurality of primary raw materials, the present process utilizes only tiuosilicic acid as the primary raw material. The overall process could be schematically illustrated as a stream of liuosilicic acid entering a treatment zone and silica and hydrogen fluoride leaving the treatment Zone.

The process of the present invention has two preferred embodiments which are illustrated in the accompanying drawing. The drawing is a schematic ow sheet illustrating both preferred embodiments. The so-called potassium bitluoride route is illustrated in the left half of the drawing and the so-called sodium biiluoride route is illustrated in the right half of the drawing. Various steps of the process as presented in the general description of the invention are also designated in the drawing in order to facilitate a complete understanding of the invention.

Step (a) Step (a) of the process of this invention is generally the same for both preferred embodiments; specifically the potassium bifluoride route and the sodium biuoride route.

In step (a) of the general description of the invention an aqueous solution of uosilicic acid is reacted with ammonia to form an aqueous solution of ammonium uoride and silica.

The reaction may be represented by the following equation:

Any suitable aqueous solution of uosilicic acid of suitable concentration may be used in the process of this invention. In the chemical treatment of fluorapatite or phosphate rock and during the evaporation of wet process phosphoric acid, a fluoride-containing gas is released, which is usually recovered as fluosilicic acid. This acid, which usually has a fluosilicic acid concentration between about 2% and about 30% by weight, may be used in the process of this invention. Industrial iiuosilicic acid from other sources may also be used. It is preferred that from about 10% to about 30% by weight tluosilicic acid may be used.

Ammonia in any suitable form and concentration may be used in this invention. Gaseous ammonia, anhydrous ammonia, and aqueous solutions of ammonia or ammonium hydroxide may be used. Ammonia is used in an amount at least sufficient to react with substantially all of the iuosilicic acid to form an aqueous solution of ammonium iiuoride and silicia. Ammonia is preferably used in stoichiometric excess of of the theoretical and more preferably within the range of from about 105% to about of the stoichiometric amount. This excess amount of ammonia raises the pH of the resultant solution to above 7.0 and preferably within the range of from about 8.5 to about 9.0. Vigorous agitation is preferred when admixing the ammonia and the fluosilicic acid. At a pH of about 8, the reaction is substantially complete and the resultant aqueous solution contains ammonium liuoride and a silica precipitate. In a preferred embodiment ofthe inventionV the major portionl and more preferably substantially all of the ammonia for stepA (a) is produced in subsequent steps of this process and recycled to step (a).

The reaction between lluosilicic'acid and ammonia to form silica and ammonium ljluoridetakes place at ami Accordingly, a modified, more efficient route using sodibient temperatures although, when desired, higher orlower temperatures may` be used. Temperatures within the range of from about F. to about 170 Y F. are suitable.

t s Sfeer y s Step (b) of the process of` this invention is generally -the same for both preferred embodiments.

`Thevsolution resulting from step (a) is an aqueous solution 4of ammonium fluoride containing 'precipitated tion of ammonium fluoride inV any suitable manner. Filtration, decantation, centrifugalV separation, intergalia, are suitable means.

' Step (c) Y v In step (c) of the processof this invention, Ythe ammonium uoride separated in step (b) and arl alkali vmetal `fluoride are introduced into atreating zone and are subjected to treating conditions to react a fluoride of ammonia with the alkali metal fluoridetoform an alkali -metal bifluoride from `said alkali Ametal iiuoride.

The ammonium ,fluoridesepara'ted in vstrep (b), which -Y is substantially Vfree of silica, may be reacted directly with f lan alkali metalfluoride providedin'an amount to react with substantially all of the Vseparated ammoniumiluo rideto form ammonia andan alkali metalbilluoride.

Of the alkali metal fluorides sodium fluoride and po- 'tassum iluoride are specifically preferred. Other alkali v 'fluorides such as Vlithium fluoride maybe used. Potas-V is, when potassium fluoride and sodium uoride are used' in the process kof lthis inventioma suliicient amount of The silica may be discarded but it" :generally has desirable physical properties vaud is recov- -ered as a product ofthe process.

the following equation:

um fluoride was developed and Vstep (c) of the process for Vthesodium bitiuoride route is preferably asV follows:

Step (c) sodium bifiuorde route The ammonium fluoride separatedin step (b) is preferably heated to form ammonium bifluoride according to zNHiFeNHrfHFJFNH, v (2) Ammonium iiuoride is also produced irl-subsequentlyl described step'(d) and a small amount in subsequently describedk step (e) inthe sodium biiuoride route of the @process of this inventionand ammonium fluoride from silica.' The silica is separated from the aqueous solup moniumbifluoride. I

,these sources (steps (d) and (e)) is preferably recycled to the heating stage of step.v (c), asis illustrated in the drawing so as to convert the ammonium liuoride to am- The ammonium fluoride of suitable conceutratiompreferably as a dilute aqueous solution of about 19% concentration, isV evaporated vand heated to a temperature VWithin the range of from about250 F. to about 410 F. to convertgthe ammonium fluoride to an ammonium bikiuoride melt.

Thetemperature range is preferably from about 300 F. Vto about 400 F. since higher yields of the biliuoride are obtainedat'temperatures withinthis pre-A ferred range. j Y

Y The reaction `according to Equation 2 also vproduces y ammonia which is preferably recycled'to step (a). In thepreferred embodiment of vthis invention sutiicient ammonia isproduced by'thereaction to provide sub- "stantiallyV all of the'arnmonia necessary for step (a) of the process. i t

AThe fluoride offammonia, specifically .ammonium biiiuoride' (NH4F-HF) produced from the ammonium flu-v '.oride (NI-IGF) is cooled in the treatment-zone and reacted in aqueous solution with sodiumtluoride (NaF) at a temperature within Vthe range of from about 50 F. to aboutj170 and preferably Within the range of from about 90 F. to about 150 F. At temperatureswithin these ranges. the ammonium biuoride and sodium liuoride react according tothe following equation:

ThatV Y the alkali metal fluoride is usually produced in subsequent steps of the process torsupply 'substantially all of the' alfy kali metal uoride needed in step (c) andit is only necessary to add make-up amounts of alkali metal fluoride into the system to take care of usual process losses. t The alkali metal fluoride is, accordingly, an'intermediate maj Y tamed during ythe reactlon.

terial which is Vconsumed andproduced in the 'overall process. In a speciiically preferred embodiment' of the invention, the alkali metal fluoride utilized in step (c) of the process is essentially only the alkalimetal fluoride' produced in subsequently described Vstep (f) and recycled inthe process. f

Thealkali metal fluoride may be `added as av dry pow# der or an aqueous slurry of suitable concentration may be used. The reaction of'ammonium fluoride with the alkali V'metalliuoride'takes place in aqueous solution fat ambient temperature although higher or lower temperatures may be used whenfdesired. Temperatures ,within the range of from about V F. to aboutp410 F. are

contemplated for this stepV of the process, the temperature'V ammonium fluoride tofform sodium biuoride and ammonia, investigations have shown that only aV 13% ,bi-

fluoride yield can be expected at reaction temperatures v within the range of from about 194 F. to about 248 F.

The temperature should, of course, be main-V Vtially all of mesh size'.`

It hask been determined that at temperatures below Y F.' the NaF-HFis lesssolublein `waterthan NaF. Accordingly, at temperatures utilized in this reaction, the NaFI-lF precipitates At temperatures above about 194 F.' the solubility relationship is reversed with NaF'HF beingy more soluble than NaF and the reaction will not proceed. Accordingly, in the sodium '.biiluoride route a maximum temperature of about 170 F. should be main- The VNaF may be introduced into the treatingzone in any suitable form, preferably as a dry solid or vas an aqueous slurry. ,Whenjintroduced as a solideither dry or asV a-'slurryfit ispreferred that the sodium fluoride be of relatively small'particle size, preferably substantially all -70 meshsize and more preferably substan- Results indicate that a 10% increase in conversion is obtained when using 100 mesh NaF as comparedto using +70 mesh Nal?.

x Step (d) sodium bifluoride route The ,product from step (c)l of` thesodiumV bifluoride route Aof the `process of this' invention is an aqueous solution vof :ammonium ybiiiuoride and ammonium iluoride containing crystals of sodium biiiuoride (NaF-HF) andY also some ammonia.` j

In step (d) ofthe process, the sodium Vbilluoride is separated from the -mixture resulting from step (c). Since the sodium ybifluorideis lin the' solid phase vas a Y precipitatel it may be separated by, inter alia, decantation, iiltration, centrifugation,etc.` or any other .method suitable for separating a rsolid from a liquid. The amymonium fluoride-containing mother liquor is preferably recycled to the heating stage of step (c) of the process as hereinbefore set forth. Some ammonia is also present and this is also preferably recycled with the ammonium fluoride.

Step (e) sodium bifluoride route The wet sodium bifluoride solid (NaF'HF) separated in step (d) still contains some occludedmother liquor and ammonium fluoride and ammonia, the specic amount, of course, depending upon the particular conditions prevailing during the liquid-solid Separation in step (d). The solid sodium bifluoride containing these impurities is, therefore, heated to a temperature high enough to dry the bifluoride and drive off or volatilize substantially all of the ammonium fluoride and ammonia but below that at which the sodium bifluoride decomposes to any appreciable degree to provide sodium bifluoride substantially free of ammonia and ammonium fluoride. Sodium bifluoride starts to decompose to a substantial degree at a temperature of about 575 F. and, therefore, the drying temperature should be below this and preferably is at a temperature Within the range of from about 212 F. to about 550 F. and more preferably within the range of from about 225 F. to about 300 F.

The ammonium fluoride and the ammonia evolved in steps (d) and (e) of the sodium bifluoride route of the invention are recycled to the heating stage of step (c) of the process. Sufficient ammonium fluoride is evolved in steps (d) and (e) to provide the ammonium fluoride necessary in step (c). Accordingly, ammonium fluoride is also an intermediate Within the overall process with ammonium fluoride being consumed in step (c) and evolved in steps (d) and (e). As hereinbefore set forth, a sufficient amount of ammonia is produced in the heating stage of step (c) to provide all the ammonia required for step (a). In a preferred embodiment of the invention, the ammonia utilized in step (a) of the process consists essentially of the ammonia produced in the heating stage of step (c) and recycled to step (a). The ammonium fluoride and ammonia streams may, of course, be subjected to purification steps before introduction into treatment zone (c) or reaction zone (a), respectively.

The heating of the ammonia-ammonium fluoride-alkali metal bifluoride mixture is an important aspect of this invention. When the alkali metal fluoride utilized in step (c) is selected from the group consisting of sodium fluoride and potassium fluoride, it is possible to heat the resulting ammonia-ammonium fluoride-alkali metal fluoride mixture from step (c) to a temperature Sulliciently high to drive off substantially all of the ammonia and ammonium fluoride while still not decomposing the particular alkali metal bifluoride. In the present integrated process, therefore, substantially all of the ammonia and ammonium fluoride may be recovered and recycled within the process While still effecting a high recovery of the fluorine values in the fluosilicic acid starting material as product hydrogen fluoride.

Steps (f), (g), (h), and (i) are generally the same for the sodium bifluoride route of the process of this invention and for the potassium bifluoride route and are subsequently described.

Step (c) potassium bifluoride route used is predominantly and preferably substantially all 'l recycle material since a sufficient amount of potassium fluoride is usually produced in subsequently described step (f) to supply substantially all of the potassium iluoride needed in step (c) and it is generally only necessary to add make-up amounts into the system to take care of usual process losses.

The potassium fluoride and ammonium fluoride from step (b) are reacted in a reaction zone at a temperature preferably Within the range of from 2007 F. to about 410 F. and more preferably within the range of from about 220 F. to about 270 F. The reaction proceeds according to the following equation:

The ammonium fluoride is preferably introduced into the reaction zone as an aqueous solution of suitable concentration, preferably a concentrated solution. In the reaction zone evaporation of Water is effected and ammonia gas is also evolved from the reaction mass. The potassium bifluoride (KF-HF) product of the reaction leaves the reaction zone as a melt.

The KF may be introduced into the treating zone as a substantially dry solid or as an aqueous slurry. When introduced as a solid either substantially dry or as a slurry, it is preferred that the potassium fluoride be of relatively small particle size, preferably substantially all of -70 mesh size particles and more preferably all of 100 mesh size particles.

Step (d) Potassium bifluoride route The product from step (c) of the potassium bifluoride route of this invention is water vapor gaseous ammonia and a melt containing predominantly potassium bifluoride and also containing some water, ammonia and ammonium fluoride. The separation of the gas may be effected in any suitable manner. The ammonia-containing gas is preferably recycled to step (a) of the process. Some ammonium fluoride is also present in this gas and the gas stream may, of course, be purified before introduction into reaction zone (a).

Step (e) potassium bifluoride route The separated potassium bifluoride melt contains some Water, ammonia and ammonium fluoride and this is removed by heating the potassium bifluoride to dryness at a temperature of from about 220 F. to about 550 F. and preferably at a temperature within the range of from about 240 F. to about 300 F. At these temperatures the ammonia and ammonium fluoride are substantially entirely volatilized but the temperature is below that at which the potassium bifluoride decomposes to a substantial or appreciable degree.

Potassium bifluoride starts to decompose t-o a substantial degree at a temperature of about 600 F. and, therefore, the drying temerature should be below about 575 F. After the drying step the potassium bifluoride is substantially free of ammonia and ammonium fluoride.

The ammonia and ammonium fluoride evolved in steps (d) and (e) of the potassium bifluoride route of the invention is recycled to step (a) of the process. This ammonia stream may, of course, be purified before introduction into zone (a). Sufficient ammonia is evolved in steps (d) and (e) to provide the ammonia necessary in step (a). Accordingly, ammonia is also an intermediate within the overall process with ammonia being consumed in step (a) and evolved in steps (d) and (e). In a preferred embodiment of the invention, the ammonia utilized in step (a) is essentially only the ammonia produced in steps (d) and (e) and recycled to step (a).

Step (t) As hereinbefore set forth, steps (if), (g), (h) and (i) are generally the same for the potassium bifluoride and sodium fluoride steps of the process. In this step, (f), the alkali metal bifluoride, substantially free of ammonia `amasar@ and ammonium liuorideis heated to decompose'the' alkali -metal billuoride to form hydrogen fluoride and alkali.

The decomposition is at temperatures ab-ove that needed for drying and the heatingy may be ,called a calcining operationas illustratedin the drawing. The solid alkaliV metal fluoride is heated to a temperature above 575 D13'.' :and isfgenerally below1000 F.,lalthough higher temperatures maybe used if desired but theyare notrgenerally necessary.' The decomposition of the-'potassium biuoride is preferablyA effected by Vcalcining `at alem- Y perature within the range of from about 625 F. to about 700 F. The decomposition of the sodium biuoride is` preferably effected by calciningat a temperaturemwithin the range of from about 575 F. to about V650 F.

ySiep '(g) The hydrogen liuoride isfproduce'd as a substantially anhydrous gas and the alkali metal fluorideileaves the calcination zone asa solid.V These'materials are, Vrtherefore, readily separatedy from each other by any suitable step .(h)

The separated alkali metal uoridexis ,recycled to step (c) of the respective process.A As hereinbefore set forth the alkali metal uoride is. preferably used in finely divided .form `and accordingly, when necessary, the-alkali Vmetalluoride isy comminuted and screened to suitable mesh size, preferably so 'that thejparticles are substantially all of --70 mesh size and more preferably of k-100 i mesh size. In a preferred embodiment of this invention substantially all of the alkaliV metalfluoride used instep (c) is provided by therecycle material with only relatively small additions of additional alkalivmetal/fluorideV being made to take care ofV processi losses.

Y Siep (u The separated hydrogen .'uoride-gasfrom stepv (g) is substantially Aanhydrous and of'high purity. The hydrogen uoride'. in this gas stream may be directly recovered or may be subjected to further purification steps before obtaining the nal hydrogen fluoride product. A suitable method for recovering the hydrogen fluoride comprises cooling the separatedgas stream containing'thle'hydrogen nXAMBLnr n Flnosilicic acid (if ab-out 23% by weightconcentration is reacted with a stoichi-ometric excess-'of concentrated Y ammonium hydroxide `solution to `produce fan aqueousV solution of ammoniumlluoride containing silica.` '125% of the theoretical amount of ammonia is Yused. andy the resultantV solutionv ha sa pH Ylof about 8.0V Vig'-V orous agitation is used during this vreaction andV a tem'-` perature of about 80 F. is-maintained. The silica was ltered vfrom the aqueous-solution of ammonium luoride. l .I

The ammoniumrfluoride'solntion of`aboutf19%A by invenand ammonia is ,drivenl oftA and recycled to-,thestripperurn fluoride solid and gaseous hydrogen fluoride.

. contacted with more ammonium liuoride.

-terialland 7parts by weight represents make-up potassium to take care of process losses. j

VIn the'reaction zone the aqueous mixture of ammonium fluoride andl potassium lluoride is subjected to evaporation and reaction at a temperature of labout l245 F. The product of this reaction is a melt containing potassium bifluoride and gas containing water rvapor and ammonia. The melt and the gas are separated from each other ywith the gas'lstream` containing ammonia being recoverediand recycled Vinto Contact with fresh iluosilicic VYacid. Y Y A f "The rneit'from the reaction Zonejis'heated in a pug milldryer at a temperature of 257 The gas Ystream from thepug miil-dryer contains additional amounts of waterv rand ammonia and some ammoniumuoride'and is also recycled into ,Contact with freshlluosilicic acicl'feed.'l The solid material from the pug-mill-'dryer is high purity potassium Vbitluoride, substantially free ofl water, Vammonia, and ammonium fluoride. This potassium biiluoride Vis Vthen calcined at a temperaturewof1662 F. during which the potassium biitiuorider decomposesl to producegaseous hydrogen fluoride and solid potassium fluoride. rhe solid potassiumfluorideis recycled to the reaction `zone and Thev gaseous hydrogen fluoride gasstream iscondensed at a temperature of 59 gF. Thefsubstantially anhydrous liquid hydrogen liuoride product represents a recovery of 82% fof the tluorinevalues inthe incomingifluosilicic acid as HF.

EXAMrfLn f rr An aqueous ammonium fluoride solution prepared from iluosilicic acid and ammoniurnlhydroxide as described in Example l is introduced Vinto a heating zoneor stripperevaporator Zone wherein the ammonium fluoride solution is heated at a temperature of 320 F. During the heating water is evaporated and the ammonium fluoride is converted into ammonium biiluoride and ammonia with the ammonia gas' passing out with the water vapor. The ammonia is recovered and recycled' into contact with fresh incomingfluosilicic acid.

The'ammonium bifluoride isremoved from the heating Zone and is -quenched with waterV to produce an Vaqueous solution of ammonium bitluoride. The aqueous solution of ammonium biiiuoride and ,-100 mesh sodiunrfluoride are introducedinto a reactor-cooler in which they are admixcd and reacted at a temperature maintainedat 122 F. For each y423 parts by weight of iiuorine in the amvmonium biliuoride material, 232parts by weight offsodium as sodium iiuoride is introduced Vinto the reactor. `2 parts' by weight of `Ibla represents make-up materialwith the rest beingfrecycle. The reaction product isa slurry containing crystals of sodium bitluoride.V The slurry is Ycentrifuged and the mother liquor y,containingammonium fluoride Vis Vrecycled to the stripper-evaporator for conversion into more-'ammonium bifluoride. The moist crystalsof Vsodium Vbiliuoridey are'dried at a temperature of 66 F. during whichy drying more ammonium fluoride evaporator..

The vdried `crystals of sodium. biuoride,wl1ich are subv stantiallyfree of ammonia and Vammonium lluoride, are

then'subjected to calcination at a temperature of 617 F. during which the sodium Vbiliuorid'e decomposes into sodiy The sodium Vtluoride isvseparated from the gas and is recycledV into contact with more, ammoniumbiliuoride.

The gaseous hydrogen iluoride-containinggas stream is v condensed ata temperature ,of 59 F. `The product is subweight concentration, is introduced into a Yreactionzone V,

wherein the ammoniumduoride vis admixed with mesh potassium fluoride. For 'each' 113 parts Vby weight of fluoride introduced into the reactiony zone 202 parts by weight of potassium as'fpotassium fluoride `is intro'- duced into the reaction zone. ,Of the 202 parts byweight of potassium, 195 parts by weight represents recycled ma-` stantially:A anhydrous liquidhydrogen iiuoride. Thisy product :represents an 82% Vrecovery of4 the kiluorineiin the lluosilicicfacid as hydrogen fluoride.

i l 'f EXAMPLnIII 1 A' solutionofVNl-MF wasprepared by adding "slowly 183.4@ of Ninon (28,9% NH3) to 30o g. of 23% tluosilicic acid'V while stirring inl a cold water bath. YSilica precipitated and the precipitated silica was separated by filtration and washed. One gram of the filtrate contained approximately 0.3 g. of NH4F.

To 25 g. of the above NH4F solution was added 19.1 g. of KF2H2O according to the reaction:

The solution became cold and NH3 was evolved. The solution was evaporated at 110 C. to dryness. 15.2 g. of crystals were obtained. Y-ray diffraction analysis of the crystals showed that .KF-HF was the major component.

The description of the invention utilized specific reference to certain process details; however, it is to be understood that such details are illustrative only and not by way of limitation. Other modifications and equivalents of the invention will be apparent to those skilled in the art from the foregoing description. Having now fully described and illustrated the invention, what is desired to be secured and claimed by Letters 'Patent is set forth in the appended claims.

We claim:

1. A process for preparing hydrogen fluoride from fluolsilicic acid which comprises the following steps:

(a) reacting lluosilicic acid with ammonia to form ammonium fluoride and silica,

(b) separating said silica from said ammonium fluoride,

(c) introducing said separated ammonium fluoride and an alkali metal fluoride into a treating zone wherein said ammonium fluoride and said alkali metal fluoride are subjected to treating conditions to react a llu- 'oride of ammonia with said alkali metal fluoride to f-orm an alkali metal billuoride,

(d) separating said alkali metal bifluoride from the mixture resulting from step (c),

(e) heating said separated alkali metal billuoride to a temperature at least sulliciently high to produce an alkali metal bifluoride substantially free of ammonia and ammonium fluoride and below that at which substantial decomposition of said alkali metal bifluoride is effected,

(f) heating the alkali metal bifluoride substantially free of ammonia and ammonium fluoride from step (e) to a temperature suiciently high to decompose said alkali metal bifluoride to form hydrogen fluoride and alkali metal fluoride,

( g) separating said alkali metal fluoride and said hydrogen fluoride produced by the heating in step (f),

(h) recycling said alkali metal fluoride separated in step (g) to step (c), and

(i) recovering said hydrogen fluoride separated in step 2. The process or" claim 1l wherein said alkali metal is potassium.

3. The method of claim 1 wherein said fluoride of ammonia comprises ammonium bifluoride and said alkali metal is sodium.

4. A process for preparing hydrogen fluoride from fluosilicic .acid 'which comprises the following steps:

(a) reacting fluosilicic acid with yammonia to form an aqueous solution of ammonium fluonide and silica,

(1b) separating said silica from lsaid aqueous solution of ammonium fluoride,

(c) introducing -said separated aqueous solution of ammonium fluoride and an alkali metal fluoride into a treat-ing zone wherein said `ammonium fluoride and said alkali metal fluoride are subjected to treating conditions includ-ing temperatures witlhin the range of from about 50 F. to about 410 F. to react a fluoride of a-mmonia with said alkali metal fluoride to form an alkali metal lbifluoride from su-bstantially all -of said alkali metal fluoride,

(d) :separating said alkali metal bifluoride from the mixture resulting from .step (c),

(e) heating said separated alkali metal bifluoride to a temperature at least sufliciently high to produce an alkali metal bifluoride substantially free of ammonia and ammonium fluoride and below that at which substantial decomposition of said alkali metal bifluoride is effected,

(f) heating the alkali metal billuoride substantially free of ammonia and ammonium fluoride from step (e) to a temperature above about 575 F. to decompose said alkali metal bifluoride to form hydrogen fluoride and alkali metal fluoride,

(g) Separating said alkali metal fluoride and said hydrogen fluoride produced by the heating in ystep (f),

(h) recycling said alkali metal fluoride separated in step (g) to step (c),and

(i) recovering said hydrogen fluoride separated in step '5. A process for preparing hydrogen fluoride from fluosilicic acid which comprises the following steps:

(a) reacting fluosilicic acid with ammonia to for-rn arnmonium fluoride and silica,

(1b) separating said silica Ifrom said ammonium fluoride,

(c) introducing .said separated ammonium fluoride and sodium fluoride into a treating zone, heating said ammonium fluoride in said treating zone to a temperature within the range of from about 250 F. to about 410 F, thereby converting said ammonium fluoride to ammonium bifluoride and ammonia, reacting said ammonium billuoride with said sodium fluoride in aqueous solution in said treating zone at a temperature within the range of from labout 50 F. to about 170 to form solid sodium 'bifluoride,

(d) separating said sodium billuoride from the mixture resulting from step (c),

(e) heating said separated sodium billuoride to a temperature within the range of from about 212 F. to about 550 F. to produce dry sodium bifluor-ide substantially free of ammonia and ammonium fluoride,

(f) heating said sodium fluoride substantially free of ammonia and ani-monium fluoride from step (e) to a temperature above about 575 F. to decompose said sodium bifluoride to form hydrogen fluoride and sodium fluoride,

(g) separating said sodium fluoride and said hydrogen fluoride produced lby the heating in step (f),

(h) recycling said sodium fluoride -separated in step (g) to step (c), and

(i) recovering said hydrogen fluoride separated in step- 6. A process 4for preparing hydrogen fluoride from fluosilicic acid which comprises the following steps:

(a) reacting fluosilicic acid with ammonia yto form ammonium fluoride and silica,

(b) separating said silica from said ammonium fluoride,

(c) introducing said separated ammonium fluoride and sodium fluoride into a treating zone, heating said ammonium fluoride in said treating zone t-o a temperature wit-hin the range of from afbout 300 F. to .about 400 lF. thereby converting said ammonium fluoride to ammonium bifluoride and ammonia, reacting said ammonium billuoride with said sodium fluoride in aqueous solution in said treating zone at a temperature within the range of from about F. to about F. to form solid sodium billuoride,

(d) separating said sodium bifluoride from the mixture resulting from step (c),

(e) heating said separated sodium bifluoride to a temperature within the range o-f from about 225 F. to about 300 F. to produce dry sodium billuoride substantially free of ammonia and ammonium fluoride,

(f) heating said sodium fluoride substantially free of ammonia and ammonium fluoride from step (e) to a temperature within the range of from about 575 lF. to about 650 F. to decompose said sodi-um bif (h) recycling said sodium luoride il l fluoride to-form --hydrogen luoride and sodi-um iluoride,

(g) separating said sodium fluoride :and said hydrogen .iiuoride produced by the heating in step (f),

separated inl step (g) to step (c), and f .(i) recovering said hydrogen yliuoride .separated in step 7..A process for preparing hydrogen fluoride from iluosilicic `acid which comprises the following steps:

(a) reacting fluosilicicacid with ammonia `to form ammonium fluoride .and silica,

(-b) separat-ingv said silica -from'said ammonium'fluoride, Y Y. Y

(c) introducing said separated ammonium liuorideand potassium uoride into a treating Vzone wherein Ysaid ammonium uoride and said potassium fluoride are reacted at a ktemperature Within the range` offrom about 220 Ffto about 270 vF. to form potassium Y biiluoride and ammonia,V l (d) separating said pot-assium bifluoride from the mixture resulting from step (c),

a temperature within therrange .of from about 240 toa|bout300 F.V to produce potassium biliuoride ride,

(if) .heating the potassium biliuoride ysubstantially free of ammonia and ammonium fluoride from. step (e)l t-o a temperaturewit-hin the range of from about l (e) heating said separated alkali Jmetal biiiuoride at V substantially free of ammonia and ammonium uoj 625 F. to about 700 F. to decompose said potas- 1 sium b-ifluoride to form hydrogen -uoride andpotassium liuoride,

(g) separating said potassium liuoride and said hydro-VV geniiuoride produced by the heating in step. (f), (h) recycling said potassium fluoride separat-edV in step (s) tostep (c), andy f (i) recovering saidhydr-ogeniiuoride separated in step l8. A. process for preparing hydrogen iiuorideffrom uosilicic acid which comprisesrfthe following steps: Y

(-a) reacting uosilicic acid with ammonia `recycled from s-tep (c) to form ammonium fluor-ide and silica,V

(b) separatingsaid silica 4from said ammonium iuo-V ride, v

(c) introducing said separated ammonium fluoride into atreating zone,A heating said yammonium fluoride land recycle ammonium fluoride from steps (d) and (e) --in said ,treating zone to atemperaturawithin the Iby converting lthe ammonium liuorideto ammoniumy biuoride Vand ammonia-recycling said ammonia to -step- (a), introducing.sodium'uorideincluding the sodium fluoride recycle of step (h) into said treating zone, reacting .said ammonium .bifluoride with vsaid 9. A process for preparing hydrogen fluoride from Luosilicic acid which rcomprises the following steps:

(a) r-eacting'uosilicic acid with ammonia recycled n from steps (d)..and.(e) to form ammonium fluoride and silica,v a v (b) separat-ing said sil-icafrom said ammonium fluo- (c) introducing said separated ammonium fluoride and potassium fluoride,r including the potassium iiuorideA f lrecycle of lstepr(h), into a treating zone wherein said ammonium liuoridey and said potassium fluoride are .reacted at av temperature within the range of from about 200 `F. to about 410 F. torformV potassium ibifluoride andv ammonia, Y f Y (d) separating saidi potassium biuoride containing some .ammoniafrom the mixture resulting from step (c) Iwhich contains .the-major portion of the ammonia and recycling said ammonia to step7(a), (e) heatingsaid separated alkali me-tal Vbiluoride at a temperature within tjhe range of from about 200 F. rto about 550` Ff. tokproduce gaseous ammonia po- Y tassium biuoride substantially free of ammonia and y ammonium iiuoride and recycling the ammonia to Step (a), t

(f) yheating the potassium bifluoridesubstantially free 'of vammornialand ammonium fluoride from step (e) to atemperature above about 600 F. to decompose v saidpotassium hiuor-ide to form hydrogen fluoride vand'potassium fluoride,

(g) separating said potassium uoride and said hydro- V genjiluforide'produced by the heating in .step (f),

(h) recycling saidy potassiumfluoride separated in step (g) to-step'(c), and

(i) recovering said hydrogen fluoride separated in step e10. A process for the production of hydrogen iiuoride om fluosi'licic acid which comprises the following steps: acting .ammonia withan laqueous solution of liuosilicic acidto form amm-onium'iiuoride and silica, separating id .silica from said ammonium fluoride, reacting said ammonium fluor-ide with potassium fluoridein aqueous solutionto form potassiumybifluoride, 4and ammonia, heating vthe resultantsolution. to drive said ammonia out of sodium uoride .in `aqueous Vsolution in said' treating zone at a temperature withinthe range of from abouti 50 F. to about 170. to form-solid. sodium'v biluoride and ammonium fluoride, Y v (d) separating said. -sodiumbiuoridecontaining some 'ammonium fluoride from the mixturel resulting from step (c)which containsrthe major portion of the ammonium fluoride andrecycling .the ammonium iluorid'egto step (c),

:about 550 F..to produce gaseous ammonium lfluoride -and dry sodium hiiluoride substantially. free of ammonia vand ammonium fluor-ide,

and recycling'the ammonium fluoride to stepV (c) ,1 u

heating said separated sodium gbifluoride to atemf peratur-e witihin the range of from YaboutfvZlZ/ F. to

e solution .and recycling said ammonial andfseparating said potassium biliuoride from the solution, heating said l separated potassium bifluoride to form potassium fluoride and hydrogen fluoride,.and vrecycling said potassium fluoride. y

in' References-Cited bythe Examiner Y .-.UNITED1STATESPATENTS l 2,865,709 .1c/5s Horn etai. V f23' 8s 12,880,060 p13/59 Campbeu s v 2.3-88 X Y, VFOREIGN :PArENrs Y i` K 1,010,504 6/57. Germany.

LBRINDIiSI, i rimary Examiner. 

1. A PROCESS FOR PREPARING HYDROGEN FLUORIDE FROM FLUOSILICIC ACID WHICH COMPRISES THE FOLLOWING STEPS: (A) REACTING FLUOSILICIC ACID WITH AMMONIA TO FORM AMMONIUM FLUORIDE AND SILICA, (B) SEPARATING SAID SILICA FROM SAID AMMONIUM FLUORIDE, (C) INTRODUCING SAID SEPARATED AMMONIUM FLUORIDE AND AN ALKALI METAL FLUORIDE INTO A TREATING ZONE WHEREIN SAID AMMONIUM FLUORIDE AND SAID ALKALI METAL FLUORIDE ARE SUBJECTED TO TREATING CONDITIONS TO REACT A FLUORIDE OF AMMONIA WITH SAID ALKALI METAL FLUORIDE TO FORM AN ALKALI METAL BIFLUORIDE, (D) SEPARATING SAID ALKALI METAL BIFLUORIDE FROM THE MIXTURE RESULTING FROM STEP (C), (E) HEATING SAID SEPARATED ALKALI METAL BIFLUORIDE TO A TEMPERATURE AT LEAST SUFFICIENTLY HIGH TO PRODUCE AN ALKALI METAL BIFLUORIDE SUBSTANTIALLY FREE OF AMMONIA AND AMMONIUM FLUORIDE AND BELOW THAT AT WHICH SUBSTANTIAL DECOMPOSITION OF SAID ALKALI METAL BIFLUORIDE IS EFFECTED, (F) HEATING THE ALKALI METAL BIFLUORIDE SUBSTANTIALLY FREE OF AMMONIA AND AMMONIUM FLUORIDE FROM STEP (E) TO A TEMPERATURE SUFFICIENTLY HIGH TO DECOMPOSE SAID ALKALI METAL BIFLUORIDE TO FORM HYDROGEN FLUORIDE AND ALKALI METAL FLUORIDE, (G) SEPARATING SAID ALKALI METAL FLUORIDE AND SAID HYDROGEN FLUORIDE PRODUCED BY THE HEATING THE STEP (F), (H) RECYCLING SAID ALKALI METAL FLUORIDE SEPARATED IN STEP (G) TO STEP (C), AND (I) RECOVERING SAID HYDROGEN FLUORIDE SEPARATED IN STEP (G). 